A great deal of attention has been focused by the packaging industry on efforts with twist caps to achieve consistent closure sealing performance and consistent levels of capping torques. A basic problem exists with the construction of the closures and with the methods and machinery used to apply caps to containers. This problem results in large variations of the torque required by the consumer to remove such caps so that some demand unusual strength or special implements while others may be so loosely applied that the effectiveness of their seal has been compromised.
With threaded closures it is typical that they are applied by capping machinery which turns the cap onto the container neck until a pre-set torque level required to assure an adequate seal is obtained. The required torque level is arrived at when the threaded engagement of the closure reaches the point where the liner or linerless sealing feature is compressed by the container neck rim to a level where the threads are so compressed against one another that they resist further engagement. Typically, the capping machine may be adjusted to provide a given capping torque level. However, most capping machines have a limited sensitivity to detect and disengage at a consistent level of torque. Some machines (e.g., those with magnetic clutches) are superior in this regard but still are lacking in consistency and are expensive. A major reason for the lack of consistency by capping machines lies in the normal variation in dimensions, surface lubricity, etc., in both caps and container neck finishes within the specifications employed for their quality control in production.
Typically, metal lug twist caps are applied to neck finishes which include a positive stop so that a limit is provided beyond which the cap cannot be twisted. The reason for such provision is that such lug engagement are short in span and, at the segment where seal compression takes place, low in pitch so that without a positive stop, the lug engagement could be exceeded and the cap would not be engaged. However, even with a positive stopping provision, wide variations in sealing force and uncapping torques are still experienced. This condition is made more severe by the high stiffness of metal and of glass containers which are typically employed for lug caps.
In general, plastic twist caps with lug engagement are seldom used where high seal integrity is needed because of the very high levels of localized stress and the resultant cold flow or creep which occurs to cause the caps to go out-of-round and to loose their sealing force. However, some use for lug type plastic caps has been developed by employing specially configured separate liners which incorporate a plug seal, a spring portion to act against the container rim and a positive stop so that very little stress is required for closure engagement, since the plug seal does not require a positive axial stress for its sealing engagement. Such caps find use for packaging dry products, primarily for prescription drugs and their design is directed towards making the closure child resistant by including a positive locking means which requires that the cap be pushed down and turned before it can be removed. Attempts to develop a one-piece closure wherein the integral liner also acts as a spring portion (see, U.S. Pat. No. 4,091,948) have been unsuccessful largely due to the fact that they have been unable to achieve the required level of flexibility and recoverable deformation in the integral liner.
In reference to the existing two-piece push-and-turn child resistant closures, problems exist with the inability of many adults to open such closures due to a lack of strength in their fingers. This fact has resulted in the use of separate caps for the same package--one child resistant and the other non-child resistant, or in the use of a two-sided cap where each side has the different feature. Both approaches are expensive and inconvenient.
Thus, known twist closures are beset with problems and drawbacks associated with their need to perform while having coacting surfaces with wide dimensional tolerance and surface lubricity, limited capping machinery sensitivity and inflexible materials resulting in specially configured and expensive liners, poor sealing performance and difficulty in opening.